A television broadcast receiver has conventionally been available to receive an analog broadcast signal such as a VHF/UHF broadcast signal via terrestrial broadcast and a signal for broadcasting satellite (BS) utilizing any broadcast satellite. A television broadcast receiver that can receives a BS digital signal or a communication satellite (CS) digital signal utilizing any broadcast satellite has recently been put to practical use. When using such the television broadcast receiver, an antenna that has been used for conventional BS analog may have been often used as it is for receiving BS digital signal.
For conventional BS analog there has been strong immunity against FM residual noise because of its FM modulation so that even if an antenna having a large amount of phase noise by its local oscillation is used, it can receive a signal properly. Thus, an antenna that has been 1 conventionally used for BS analog may be used for BS digital as it is, but when the above antenna having a large amount of phase noise is used for BS digital, there has been weaker immunity against noise than that in a case of BS analog because BS digital is modulated using an eight-phase-shift-keying (8PSK) modulation scheme, so that its receiving characteristics may be deteriorated.
FIG. 1 is a diagram for illustrating an arrangement for signal points of a baseband signal by the 8PSK modulation. In this 8PSK modulation, eight phase equalization points are used to transmit information of three bits for every symbol. If numbers of the phase equalization in PSK is increased, an amount of information for every symbol is increased, but possibility that it is erroneously received as an adjacent signal point under the influence of noise etc. is also increased.
Really, if local oscillator may oscillates normally, signal points of the baseband signal spreads from any one based on random noise, thereby resulting in low Carrier to Noise (C/N) ration. FIG. 2 is a diagram for illustrating oscillation spectrum when a local oscillator oscillates normally. FIG. 3 is a diagram for illustrating a situation of signal points when the random noise occurs. If C/N ratio is decreased, an error amount of transport stream of demodulation IC output is increased so that a bit-error rate may be deteriorated to finally reach any deterioration of receiving characteristics such that block noise can occur in a screen.
FIG. 4 is a diagram for illustrating oscillation spectrum when high phase noise is included. FIG. 5 is a diagram for illustrating a situation of signal points when phase noise occurs. As using the antenna having a large amount of noise as described above, the phase noise is overlapped with the received signal as it is, but signal points of phase mapping of the baseband signal spread in a circumferential direction as shown in FIG. 5 because phase noise includes noise component having any distortion in a frequency component thereof. This is because probability that demodulator may erroneously determine it as an adjacent signal point is increased since there is a shorter distance between the signal points of the baseband signal, thereby resulting in augmentation of errors in output transport stream (TS).
Constellation in a case where phase noise in a local oscillator is impaired has been recently studied and it has been also reported that as disclosed in, for example, “QAM FOR TERRESTRIAL AND CABLE TRANSMSSION” by David A. Bryan, IEEE Trans. on Consumer Electronics, Vol. 41. No.3, August 1995 (see pages 385 and 86 and FIG. 4), signal points of phase mapping of the baseband signal spread in a circumferential direction.
Alternatively, a local oscillator of a converter has been normally operated from the start of a period of purchase time for an antenna but deterioration of oscillation characteristics of the local oscillator due to long-term deterioration, with it is exposed to the outside air for a long time, such as moisture entry thereinto due to deterioration in its waterproof and a loose contact of ground due to repetition of temperature changes may occur, so that parasitic oscillation as shown in FIG. 6 can occur. FIG. 6 is a diagram for illustrating oscillation spectrum when the parasitic oscillation occurs. According to a test result, it is appreciated that as shown in FIG. 7, signal points of phase mapping of the baseband signal spread in a radial direction. In this case, because of the same reasons like the above, this becomes a factor in deterioration of its receiving characteristics.